Electrode, method of making same, photoelectric transfer element, method of manufacturing same, electronic device and method of manufacturing same

ABSTRACT

An electrode is composed of a carbon carrying a metal and a binder polymer, and it is used as a counter electrode of a dye-sensitized solar cell. The metal carried by carbon is at least one kind of metal selected from the group consisting of Pt, Ru, Co, Ti, Ni, Al and Au. The carbon is needle-like carbon, fullerene, carbon nanotube, conductive carbon black, or the like, and its specific surface area is equal to or larger than 100 m 2 /g.

CROSS REFERENCES TO RELATED APPLICATIONS

The present invention is a divisional of U.S. application Ser. No.10/523,483, filed Nov. 10, 2005, which claims priority under 35 U.S.C.371 and 37 CFR 1.495 to International Application Serial No.PCT/JP04/007559, filed May 26, 2004, which, in turn, claims priorityunder 35 U.S.C. §119 to Japanese Patent Application No. JP 2003-161767,filed in the Japanese Patent Office on Jun. 6, 2003, the entire contentsof which being incorporated herein by reference.

TECHNICAL FIELD

This invention relates to an electrode, a method of forming same, aphotoelectric transfer element, a method of manufacturing same, anelectronic device and a method of manufacturing same, which are suitablefor application to solar cells using semiconductor electrodes made ofsemiconductor nanoparticles.

BACKGROUND ART

Heretofore, solar cells of various materials have been examined. Amongthem, a number of solar cells made by using silicon have beencommercially available. They are roughly classified to crystallinesilicon solar cells using single crystal silicon or polycrystal siliconand amorphous silicon solar cells.

In crystalline silicon solar cells, photoelectric transfer efficiency,which is the performance of converting light (sun) energy to electricalenergy, is higher than that of amorphous silicon solar cells. However,since crystalline silicon solar cells need much energy and time forcrystal growth, they are disadvantageous in terms of the cost because ofthe low productivity.

Amorphous silicon solar cells are advantageous in higher lightabsorption, wider selectable range of substrates and easier enlargementof the scale. However, photoelectric transfer efficiency of amorphoussilicon solar cells is lower than that of crystalline silicon solarcells. Furthermore, although amorphous silicon solar cells are higher inproductivity than crystalline silicon solar cells, they need anevacuation process for the manufacture similarly to crystalline siliconsolar cells and still impose a load to the manufacturing process interms of equipment.

On the other hand, there have been long researches of solar cells usingorganic materials to solve the above problems. However, many of themhave poor photoelectric transfer coefficient as low as 1% and have notbe turned into practical use.

Among them, dye-sensitized solar cells introduced on Nature 353, 737,(1991) are remarked because they have been proved enable to realizephotoelectric transfer efficiency as high as 10% and are consideredmanufacturable economically. The general structure of dye-sensitizedsolar cells is shown in, for example, Japanese Patent Laid-openPublication No. JP-H01-220380.

As counter electrodes of dye-sensitized solar cells, platinum (Pt)exhibiting small oxidation-reduction overvoltage of redox pairs has beenmainly used conventionally. However, there are other reports on a methodof using simplex carbon (The Electrochemical Society of Japan, Papersfor 2002 Spring Symposium, Imoto et al., 3119) and a method of usingelectrically conductive polymers (The Electrochemical Society of Japan,Papers for 2002 Autumn Symposium, Yanagida et al., 2E30) as well.

Iodine is known as specifically adheres onto platinum (Pt) and enablingrealization of quick charge transfer (Mol. Cryst. Liq. Cryst. (1985)121, 285)

Further known is a method of preparing TiO₂ paste in which titaniumoxide (TiO₂) particles are dispersed (“Latest Technology ofDye-sensitized Solar Cells” by Hironori Arakawa, CMC, pp. 45-47 (2001))

Furthermore, a method of preparing carbon carrying Pt is known as well(Japanese Patent Laid-open Publication No. JP-H05-174838).

As referred to above, Pt has been mainly used as the counter electrode.However, charge transfer velocity on Pt electrodes is not alwayssatisfactory. In addition, although the foregoing documents report theuse of simplex carbon or electrically conductive polymers, chargetransfer velocity in these methods is still insufficient.

It is therefore an object of the invention to provide an electrodehigher in electron transfer velocity than Pt, simplex carbon,electrically conductive polymers, and so on, a method of manufacturingsame, a photoelectric transfer element using this electrode, a method ofmanufacturing same, an electronic device using the same electrode and amethod of manufacturing same.

DISCLOSURE OF INVENTION

The Inventor carried out various experiments and researches to solve theaforementioned problems involved in the conventional techniques, andfound that the use of an electrode of a carbon carrying a metal insteadof simplex carbon as the counter electrode is effective for realizationof quick charge transfer on the counter electrode of a dye-sensitizedsolar cell. Thus, the Inventor has reached the present invention.

A method of using an electrode having high specific surface area isgenerally known as a method for realizing quick charger transfer. Acarbon carrying a metal used in the electrode according to the inventionenables realization of high charge transfer by high specific surfacearea as a result of using carbon and the catalytic effect of the metalcarried as nanoparticles. In combination with the conventional knowledgethat iodine specifically adheres onto Pt and enables realization ofquick charge transfer as described in one of the above documents,realization of high charge transfer is expected especially in anelectrode containing carbon of high specific surface area and Pt.

The present invention has been made through the studies mentioned above.

That is, according to the first aspect of the invention toward solutionof the aforementioned problems, there is provided an electrodecomprising a carbon carrying a metal and a binder.

According to the second aspect of the invention, there is provided amethod of making an electrode, comprising:

forming a mixture of a carbon carrying a metal and a binder on anelectrically conductive substrate.

According to the third aspect of the invention, there is provided aphotoelectric transfer element using an electrode composed of a carboncarrying a metal and a binder.

According to the fourth aspect of the invention, there is provided amethod of manufacturing a photoelectric transfer element, comprising thestep of:

making an electrode by forming a mixture of a carbon carrying a metaland a binder on an electrically conductive substrate.

In the third and fourth aspects of the invention, the photoelectrictransfer element is typically configured to place the electrode as thecounter electrode in an opposed relation with a transparent electricallyconductive substrate and to include a semiconductor layer and anelectrolytic layer between them. The photoelectric transfer element istypically configured as dye-sensitized solar cell. However, thephotoelectric transfer element may be either a solar cell other thandye-sensitized solar cell or a photoelectric transfer element other thansolar cells.

Configuration of a carbon carrying a metal may be selected from variousconfigurations. For example, the carbon may be needle-like carbon,fullerene, carbon nanotube (including a carbon nanohorn), electricallyconductive carbon black (such as KETJENBLACK (trademark) or acetyleneblack), or the like. These various types of carbon may exhibit stillbetter effects when they are polymerized or a functional group isintroduced. Specific surface area of the carbon is preferably large toenhance the charge transfer speed on the electrode. The specific surfacearea of the carbon is typically at least 100 m²/g, or preferably equalto or larger than 300 m²/g. It is known that an electrode made by usingthe carbon has surface area as large as 100 times or more of theprojected area. Grain size of this carbon is typically equal to orsmaller than 100 nm. However, to make it easier to form the electrode,it is acceptable to mix carbon having a larger grain size.

There is no specific limitation for the grain size of the carbon withthe larger grain size. However, considering that the maximum thicknessof the electrode is 20 to 30 μm, its maximum size will be in the orderof 1 to several μm.

The metal carried by the carbon is at least one kind of metal selectedfrom the group consisting of platinum (Pt), ruthenium (Ru), cobalt (Co),titanium (Ti), nickel (Ni), aluminum (Al) and gold (Au), for example.Candidates of the metal also include mixtures or compounds of thosemetals. The carbon carrying the metal assures better electrodecharacteristics than simplex carbon, and the electrode characteristicscan be improved by increasing the amount of the carried metal. To obtaina noticeable effect of improvement of the characteristics as comparedwith simplex carbon, the carried metal is preferably included by theamount of at least 5 weight percent (wt %) of the carbon. In case theprecious metal Pt is used as the carried metal, it is desirable tominimize its amount from the viewpoint of minimizing the cost. Thus, theamount of the metal is preferably limited not to exceed 15 wt %, forexample.

In the electrode summarized above, it will be possible to useelectrically conductive polymer in addition to the metal in order toenhance the favorable characteristics as the counter electrode. To formthe electrically conductive polymer, various methods can be used, suchas a method of casting liquid solutions of macromolecule monomers oncarbon and thermally polymerizing them, a method of directly casting apolymer solution, or a method of forming it by electrolyticpolymerization in a monomer solution, although they are not limitative.

In the first to fourth aspects of the invention, the electrode composedof the carbon carrying the metal and the binder is typically formed onan electrically conductive substrate. The electrically conductivesubstrate is typically made of glass, polymer film or metal, forexample. More broadly, however, usable conductive substrates areexplained below. The electrically conductive substrate may be either asimplex conductive substrate or a combination of a conductive ornon-conductive support substrate and a conductive film formed on thesupport substrate. The conductive substrate is typically transparent. Inthis case, the substrate is typically an electrically conductivetransparent substrate in its entirety, or may be made by forming atransparent conductive film on a conductive or non-conductivetransparent support substrate. There is no specific limitation for theconductive substrate, and various materials are usable. Especially inthe photoelectric transfer element, the conductive substrate preferablyhas high blocking capability against intruding moisture and gas fromoutside the photoelectric transfer element, high resistance to thesolvent and high weather resistance. Examples of such substrates aretransparent inorganic substrates of quartz, glass, or the like,transparent plastic substrates of polyethylene terephthalate,polyethylene naphthalate, carbonate, polystyrene, polyethylene,polypropylene, polyphenylene sulfide, polyvinylidene fluoride,tetraacetyl cellulose, phenoxy bromide, aramid, polyimide, polystyrene,polyarylate, polysulfone, polyolefin, and so forth. However, materialsof the substrate are not limited to them. Taking easier workability andlighter weight into account, a transparent plastic substrate ispreferably used as the conductive substrate. There is no specificlimitation for the thickness of the conductive substrate. Any thicknessis selectable depending upon the light transmittance, blockingcapability between the inside and the outside of the photoelectrictransfer element, and other factors.

The smaller the surface resistance, the better the conductive substrate.More specifically, the surface resistance of the conductive substrate ispreferably 500 Ω/□ or less, and more preferably 100 Ω/□ or less. In casethe conductive substrate is made by forming a conductive film on asupport substrate, known materials can be used. Examples of suchmaterials are indium-tin complex oxide (ITO) fluorine-doped SnO₂ (FTO)and SnO₂. They are not limitative, but two or more of them can be usedin combination. For the purpose of reducing the surface resistance ofthe conductive substrate and thereby enhancing the collectingefficiency, a pattern of metal wiring of high conductivity can be madeon the conductive substrate.

Also as the substrate on which the semiconductor layer is formed,various substrates of the materials and structures explained above canbe used.

The binder for forming a complex with the carbon carrying the metal maybe selected from known materials such as various kinds of pitch, rubber,plastic resin, and so on. The binder is preferably insoluble toelectrolytes. More specifically, examples of materials usable as thebinder include fluoroplastics such as polyvinylidene fluoride (PVDF),polytetrafluoro ethylene (PTFE), tetrafluoroethylene hexafluoropropylenecopolymer (FEP), tetrafluoroethylene perfluoroalkylvinylether copolymer(PFA), ethylene tetrafluoroethylene copolymer (ETFE),polychlorotrifluoroethylene (PCTFE), ethylene chlorotrifluoroethylenecopolymer (ECTFE), polyvinyl fluoride (PVF), etc.; vinylidenefluoride-based fluororubbers such as vinylidene fluoridehexafluoropropylene-based fluororubbers (VDF-HFP-based fluororubbers),vinyliden fluoride hexafluoropropylene tetrafluoroethylene-basedfluorororubbers (VDF-HFP-TFE-based fluororubbers), vinyliden fluoridepenthafluoropropylene-based fluororubbers (VDF-PEP-based fluororubbers),vinylidene fluoride penthafluoropropylene tetrafluoroethylene-basedfluororubbers (VDF-PEP-TFE-based fluoro rubbers), vinylidene fluorideperfluoro-methylvinylether tetrafluoroethylene-based fluororubbers(VDF-PFMVE-TFE-based fluororubbers, vinylidene fluoridechlorotrifluoroethylene-based fluororubbers (VDF-CTFE-basedfluororubbers), etc.; tetrafluoroethylene propylene-based fluororubbers(TFE-P-based fluororubbers); tetrafluoroethyleneperfluoroalkylvinylether-based fluororubbers; thermoplasitcfluororubbers (such as DAI-EL thermoplastic manufactured by DaikinIndustries, Ltd.); polyethylene glycol (PEG); ethylene propylene dienemonomer (EPDM); and cellulose such as carboxymethyl cellulose (CMC).Among them, polyvinylidene fluoride (PCDF) can be dissolved especiallyby solvent, and can be easily mixed with slurry. In addition, this isdesirable because of stability to light, heat, etc. The binder may be acombination of two or more of the above-mentioned materials.

The binder is typically prepared and used by dissolving or dispersing apowdered binder material in a solvent. However, powder of a bindermaterial may be used directly without using a solvent. There is nospecific limitation for the solvent to be used, and any appropriatesolvent may be selected from various kinds of solvents including water,methylethyl ketone, cyclohexanone, isophorone, N-methylpyrrolidone,N,N-dimethyl formamide, N,N-dimethyl acetamide, toluene, and so forth,depending upon the purpose.

Regarding the quantity of the solvent to be added, there is no specificlimitation provided sufficient adhesive force to a base material used ina later manufacturing process or in operation is obtained. Typically,however, quantity of the binder to be added is 5 wt % or more, or morepreferably 15 wt % or more of the carbon. Too little binder will resultin insufficient adherence to the base material, and too much binder willresult in insufficient characteristics of the electrode.

Regarding the way of forming the mixture of the carbon carrying themetal and the binder on the conductive substrate, there is not specificlimitation. Any appropriate one of known methods such as metal maskprinting, electrostatic coating, dip coating, spray coating, rollcoating, doctor blade technique, gravure coating, screen printing, andso on, may be used. After that, rolling treatment by a flat press,calendar roll, or the like, may be added, if necessary.

In the photoelectric transfer element, the semiconductor layer formedbetween the transparent conductive substrate and the above-explainedelectrode as the counter electrode is typically composed ofsemiconductor nanoparticles. Usable materials of the semiconductornanoparticles are element semiconductors represented by silicon as wellas various compound semiconductors, oxide semiconductors, and so forth.These semiconductors are preferably n-type semiconductors in whichelectrons in the conduction band behave as carriers and provide an anodecurrent. Examples of these semiconductors are metal oxides such as TiO₂,MgO, ZnO, WO₃, Nb₂O₃, TiSrO₂ and SnO₂. Among them, TiO₂ (especially ofthe anatase structure) is especially desirable. However, usablesemiconductors are not limited to those examples, and any complexcombining two or more of them may is also usable.

There is no specific limitation to the grain size of the semiconductornanoparticles. However, it is preferably 1 to 200 nm and more preferably5 to 100 nm in average grain size of primary particles. It is alsopossible to mix semiconductor nanoparticles having a larger averagegrain size with the semiconductor nanoparticles having theaforementioned average grain size to scatter incident light by thesemiconductor nanoparticles having the larger average grain size scatterand thereby enhance the quantum yield. In this case, the average grainsize of the semiconductor nanoparticles added is preferably 20 to 500nm.

The sensitizing dye carried by the semiconductor nanoparticles may beselected freely provide it brings about a sensitizing function. Examplesof usable dyes include xanthene-based dyes such as bipyridine,phenanthrene derivatives, rhodamine B, rose bengal, eosin andErythrocin; cyanine-based dyes such as quinocyanine and cryptocyanine;basic dyes such as phenosafranine, Capri blue, thiocin and methyleneblue; porphyrin-based compounds such as chlorophyll, zinc porphyrin andmagnesium porphyrin; azo dyes; phthalocyanine compounds; coumarin-basedcompounds; anthraquinone-based dyes; and polycyclic quinone-based dyes.Complex with a metal such as ruthenium (Ru), zinc (Zn), platinum (Pt) orpalladium (Pd) is also usable as the sensitizing dye. In particular, Rubipyridine complex compound is preferable because of its high quantumyield. Any mixture of two or more kinds of those substances is alsousable as the sensitizing dye.

The sensitizing dye may be carried by the semiconductor nanoparticlelayer in any form or manner. For example, a typical method is todissolve the sensitizing dye in a solution such as various kinds ofalcohol, various kinds of nitrile, nitromethane, halogenatedhydrocarbon, various kinds of ether, dimethyl sulfoxide, various kindsof amide, N-methylpyrrolidone, 1,3-dimethyl imidazolidinone, 3-methyloxazolidinone, various kinds of ester, various kinds of carbonic acidester, various kinds of ketone, hydrocarbon, water, and so on, nextimmerse the semiconductor nanoparticle layer therein, or coating thesemiconductor nanoparticle layer with the dye solution. Also employableis a method of dissolving the sensitizing dye in a paste in whichsemiconductor nanoparticles are dispersed, then coating a layer withsemiconductor particles previously carrying the sensitizing dye, andpress-molding the layer. In this case, amount of sensitizing dyemolecules absorbed by each semiconductor nanoparticle is preferably 1 to10000 molecules, or more preferably 1 to 3000 molecules. The dyemolecules may be either monomolecular or in form of association. Tocontrol the association, temperature and pressure for the carrying maybe changed. To reduce association of sensitizing dye moleculesthemselves, a kind of carboxylic acid such as deoxycholic acid may beadded. An ultraviolet absorber may be additionally used as well.

The electrolytic layer contains at least one kind of substancesreversibly changeable in state between oxidation and reduction (redoxsystem, which is dissolved in an electrolyte. Examples of the redoxsystem are halogen system such as I-/I₃- and Br-/Br₂, quasi-halogensystem such as quinone/hydroquinone and SCN-/(SCN)₂, iron (II) ion/iron(III) ion, copper (I) ion/copper (II) ion, and so on, although notlimitative to them.

The electrolyte may be a liquid electrolyte. Alternatively, theelectrolyte may be prepared in form of a gel electrolyte, polymericsolid electrolyte, inorganic solid electrolyte, or the like, whichcontains a liquid electrolyte in a polymeric substance. Morespecifically, the electrolyte may be a combination of iodine (I₂) and ametal iodide or an organic iodide, a combination of bromine (Br₂) and ametal bromide or an organic bromide, a metal complex such asferrocyanide/ferricinium ion, a sulfur compound such as sodiumpolysulfide or alkylthiol/alkyl disulfide, viologen dye,hydroquinone/quinone, or the like. Preferable cations of theabove-mentioned metal compounds are Li, Na, K, Mg, Ca, Cs, etc.Preferable cations of the above-mentioned organic compounds arequaternary ammonium compounds such as various kinds of tetra alkylammonium, various kinds of piridinium, various kinds of imidazolium, andso forth. However, the cations are not limited to these materials, butcombinations of two or more kinds of them may be used. Among them,electrolytes made by combining 12 with an ionic liquid such as LiI, NaI,imidazolium iodide, quaternary ammonium iodide, or the like.Concentration of the electrolyte salt is preferably 0.05M to 5M, or morepreferably 0.2M to 1M. Concentration of 12 and Br₂ is preferably 0.0005Mto 1M, or more preferably 0.001M to 0.1M. To enhance the releasevoltage, various kinds of additives such as 4-tert-butyl pyridine orcarboxylic acid may be added.

Usable as the solvent of the electrolyte are, for example, the nitrilesystem including acetonitrile, carbonate system including propylenecarbonate and ethylene carbonate, gamma butyrolactone, pyridine,dimethyl acetamide, other polar solvents, cold-melting salts such asmethyl propyl imidazolium-iodine, and their mixtures. Commoner solventsof the electrolyte are water, various kinds of alcohol, ether, ester,ester carbonate, lactone, carboxylic ester, phosphoric triester,heterocyclic compounds, various kinds of nitrile, ketone, amide,nitromethane, halogenated hydrocarboon, dimethyl sulfoxide, sulfolane,N-methylpyrrolidone, 1,3-dimethyl imidazolidinone, 3-methyloxazolidinone and hydrocarbon. Mixtures of two or more of thesematerials are also usable. Furthermore, ionic liquids of the tetraalkylsystem, pyridinium system, quaternary ammonium salt of the imidazoliumsystem can be used as the solvent.

The electrolyte may additionally contain a supporting electrolyte wherenecessary. Examples of the supporting electrolyte are inorganic saltssuch as lithium iodide and sodium iodide, and molten salts such asimidazolium and quaternary ammonium.

The photoelectric transfer element may be made by any method. Forexample, the electrolytic composite can be in a liquid form or may begelated inside the photoelectric transfer element. In case theelectrolytic composite is in a liquid form before it is introduced, thesemiconductor electrode and the counter electrode are opposed to eachother, and the part of the substrate not covered by the semiconductorelectrode is sealed such that these two electrodes do not contact. Inthis case, size of the gap between the semiconductor electrode and thecounter electrode is normally 1 to 100 μm or more preferably 1 to 50 μm,although it is not limitative. If the distance between the electrodes isexcessively long, photoelectric current will decreases due to a decreaseof the electrical conductivity. The sealing may be formed by any method,but the use of a light-resistant, insulative and material reactive tolight, insulative and dampproof material is preferable. Various weldingmethods, epoxy resin, ultraviolet-curing resin, acrylic adhesive, EVA(ethylene vinyl acetate), ionomer resin, ceramic, heat-welding film,etc. can be used. An inlet for introducing the solution of theelectrolytic composite is required. Location of the inlet preferablyavoids the counter electrode, although it is not limitative. Thesolution may be introduced by any method, but a method of introducingthe solution into the cell already sealed except an inlet. In this case,it is easy to pour several drops of the solution into the inlet andintroduce them into the cell by capillary phenomenon. Introduction ofthe solution may be conducted under a reduced pressure or a heatedcondition where necessary. After the required amount of the solution isfully introduced inside, an extra amount of the solution remaining inthe inlet is removed, and the inlet is sealed. The inlet may be sealedby any method. If necessary, it can be sealed by bonding a glass plateor a plastic substrate with a sealing agent. In case the electrolyte isa gel electrolyte using a polymer, or the like, or a total solidelectrolyte, the polymer solution containing the electrolytic compositeand a plasticizer is vaporized and removed on the semiconductorelectrode carrying the dye by a casting method. After the plasticizer isfully removed, the inlet may be sealed by the same method. The sealingis preferably done in an atmosphere of an inactive gas or under areduced pressure by using a vacuum sealer, for example. After thesealing, heating and pressing may be carried out to assure sufficientimpregnation of the electrolyte into the semiconductor nanoparticlelayer.

The photoelectric transfer element can be made in various non-limitativeforms depending upon the use.

The aforementioned electrode comprising the carbon carrying the metaland the binder, as well as the method of forming the electrode byforming the mixture of the carbon carrying the metal and the binder onthe conductive substrate, is applicable not only to photoelectrictransfer elements but also to all electronic devices using electrodes.

Thus, the fifth aspect of the invention is an electronic device using acarbon carrying a metal and a binder.

The sixth aspect of the invention is a method of manufacturing anelectronic device comprising the step of forming an electrode by forminga mixture of a carbon carrying a metal and a binder on a conductivesubstrate.

The explanation and discussion made in conjunction with the first tofourth aspects of the invention are applicable to the fifth and sixthaspects of the invention provided they are consistent to their natures.

According to the invention having the above-summarized configuration,since a carbon carries a metal, the high specific surface of the carbonand catalytic action of the carried metal greatly enhance the chargetransfer velocity on the electrode.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of the substantial part of adye-sensitized solar cell according to an embodiment of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the invention will now be explained below withreference to the drawing.

FIG. 1 shows a dye-sensitized solar cell embodying the invention.

As shown in FIG. 1, the dye-sensitized solar cell comprises two partsopposed to each other via a predetermined distance. One of these partscomprises a transparent electrode and a metal oxide semiconductor layer3 (semiconductor electrode) sequentially formed on a transparentsubstrate 1. The other parts comprises an electrode 5 and a counterelectrode 6 sequentially formed on a substrate 4. In the space betweenthose two parts, an electrolyte layer 7 is formed. The entirety ishoused sealed in a case 8. Instead of being housed in the case 8, theentirety may be sealed with a resin. The transparent electrode 2 and thecounter electrode 6 are connected by a conducting wire to form a currentcircuit 10 with an ammeter 9. In this structure, light impinges themetal oxide semiconductor layer 3 from through the transparent substrate1.

The electrode 5 may be omitted. To enhance the adhesion between thecounter electrode 6 and the electrode 5, or the adhesion between thecounter electrode and the substrate 4 in case the electrode 5 isomitted, a layer of Cr or the like may be interposed between them. Theelectrode 5 and the counter electrode 6 may be formed integral. Theelectrode 5 is made of, for example, glass, transparent conductiveglass, metal, polymer film, or the like, although not limitative tothese examples. However, material of the electrode 5 is preferablyselected from those not reacting the electrolyte layer 7 when contactingit through pinholes in the counter electrode 6, if any.

The transparent substrate 1 and the substrate 4 can be selected fromexamples shown above, depending upon given requirements. For example,they may be glass substrates, transparent plastic substrate, or thelike.

The transparent electrode 2 is made of a transparent conductivematerial. More specifically, the transparent electrode 2 may be a singlefilm of ITO most widely known as a transparent conductive materialnon-doped or doped with an element such as Zr, Hf, Te, F, or the like,or may be a multi-layered structure of such a film and a film of anothertransparent conductive material. The multi-layered structure may be madeby stacking a film of a metal such as Au, Ag or Cu between ITO films,for example, although it is not limitative.

The metal oxide semiconductor layer 3 includes metal oxide semiconductornanoparticles sintered on the transparent electrode 2, for example.Material of the metal oxide semiconductor layer 3 can be selected fromthe aforementioned materials, for example, depending upon givenrequirements.

A sensitizing dye (not shown) is carried on the metal oxidesemiconductor layer 3 to sensitize it. The sensitizing dye can beselected from the aforementioned materials, for example, depending upongiven requirements.

The electrolyte, redox system, solvent, etc. of the electrolyte layer 7can be selected from the aforementioned materials, for example,depending upon give requirements. The electrolyte layer 7 may contain asupporting electrolyte, if necessary. The supporting electrolyte can beselected from the aforementioned materials, for example, depending upongive requirements.

In this case, the counter electrode 6 comprises carbon carrying a metaland binder polymer. Essentially, the carbon carrying the metal may takeany form. Preferably, however, it is in form of carbon nanotubes(including carbon nanohorns), acicular carbon, KETJENBLACK (trademark),acetylene black, or the like. The metal carried by the carbon may be atleast one kind of metal selected from the group consisting of Pt, Ru,Co, Ti, Ni, Al and Au. Amount of the metal carried by the carbon is 5 wt% or more, for example, relative to the carbon. The binder polymer maybe selected from the aforementioned materials, for example, dependingupon given requirements.

The counter electrode 6 composed of carbon carrying a metal and binderpolymer can be made by the following process, for example. The followingexplanation is made as using Pt as the metal.

First, a carbon carrying Pt is prepared by, for example, the methoddescribed in Patent Document 2. That is, 1 g of chloroplatinic acid(H₂PtCl₆) is dissolved in 250 cc of distilled water, and 10.6 cc of 30wt % hydrogen peroxide is added and stirred. After that, thiosulfuricacid Na₂S₂O₄ water solution 60 g/1,106 cc is added and stirred toprepare colloidal Pt. Also prepared is a carbon-dispersed solution inwhich 1 weight part of carbon for previously carrying colloidal Pt iswell dispersed in 100 weight part of distilled water by an ultrasonicagitator. The carbon-dispersed solution, heated to 60° C., is instilledto have the carbon carry Pt catalyst. Then, it is filtered by suctionfiltration and cleaned well. Thus, the carbon carrying Pt is obtained.

Subsequently, binder polymer (such as N-methyl pyrrolidone (NMP) andPVDF, for example) is added to the carbon carrying Pt, and they aremixed until the mixture becomes uniform. The mixture is next coated onthe electrode 5 by screen printing, blade coating, or the like; thendried by heating and/or depressurization; and pressed. Thus, the counterelectrode 6 comprising the carbon carrying Pt and the binder polymer isobtained. When the mixture is pressed, heating may be additionally used.In this process, temperature is preferably controlled to be equal to orhigher than the glass transition point of the binder polymer used, suchas PVDF.

In case the binder is a water-soluble polymer such as cellulose or PTFE,while water and isopropanol are gradually added, the binder polymer of50 wt % relative to carbon, for example, is mixed until the mixturebecomes uniform. Then, the mixture is coated on the electrode 5 byscreen printing, blade coating, or the like; then dried by heatingand/or depressurization; and pressed. Thus, the counter electrode 6comprising carbon carrying a metal and binder polymer is obtained. Hereagain, temperature is preferably controlled to be equal to or higherthan the glass transition point of the binder polymer used, such asPVDF.

Operation mechanism of the dye-sensitized solar cell is as explainedbelow.

Light entering through the transparent substrate 1 excites thesensitizing dye carried by the metal oxide semiconductor layer 3, andthe excited sensitizing dye quickly delivers electrons to the metaloxide semiconductor layer 3. On the other hand, the sensitizing dyelosing electrons receives electrons from ions of the electrolyte layer 7that is a carrier transfer layer. Molecules having delivered electronsagain receive electrons in the counter electrode 6. In this manner, acurrent flows between the electrodes. Since the transparent electrode 2and the counter electrode 6 are connected by a current circuit 10 asalready explained, electrons generated flow into the counter electrode 6via the metal oxide semiconductor layer 3. As a result, electric energycan be obtained between the transparent electrode 2 and the counterelectrode 6.

As explained above, according to the embodiment, since the solar celluses the counter electrode 6 composed of carbon carrying a metal such asPt and binder polymer, it is possible to enhance the charge transfervelocity on the counter electrode 6 by high specific surface area of thecarbon and the catalytic action of Pt or other metal carried by thecarbon as compared with conventional solar cells using Pt, simplexcarbon or conductive polymer as the counter electrode 6. As a result, itis possible to enhance the photoelectric transfer property of thedye-sensitized solar cells.

Some examples are explained below.

EXAMPLE 1

TiO₂ nanoparticles were used as the semiconductor nanoparticles. TiO₂paste was prepared in the following manner with reference to Non-patentDocument 5.

First, 125 ml of titanium isopropoxide was slowly instilled into 750 mlof 0.1 M nitric acid water solution at the room temperature while thesolution is stirred. After completion of the instillation, the solutionwas transferred to a constant temperature bath of 80° C. and stirred foreight hours. Thus, a cloudy semitransparent sol solution was obtained.The sol solution was allowed to cool to the room temperature, thenpassed through a glass filter, and measured up to 700 ml. The solsolution obtained was transferred to an autoclave, then treated byhydrothermal processing at 220° C. for 12 hours and thereafter put underdispersion processing by supersonic processing for one hour. After that,the solution was concentrated by an evaporator at 40° C. to adjust thecontent of TiO₂ to 11 wt %. Then, polyethylene oxide (PEO) of molecularweight 500,000 was added to the concentrated sol solution, and they weremixed uniformly by a planet ball mill. Thus, TiO₂ paste increased inviscosity was obtained.

The TiO₂ paste was coated over the area of 0.7 cm×0.7 cm on FTO used asthe transparent electrode 2 by screen printing. Thereafter, the FTO washeld in a nitrogen atmosphere at 450° C. for 60 minutes, and nanoporousTiO₂ electrodes were sintered on the FTO substrate (sheet resistance;15Ω/□).

After that, the FTO substrate was immersed in dehydrated ethanolsolution in which 0.5 mM ofcis-bis(isothiocyanate)-N,N-bis(2,2′-dipyridil-4,4′di-carbonicacid)-ruthenium(II) dihydrate and 20 mM of deoxycholic acid weredissolved for 12 hours to have it absorb the sensitizing dye. Theelectrode was next cleaned sequentially in ethanol solution of4-tert-butyl pyridine and dehydrated ethanol, and thereafter dried inthe dark.

The counter electrode 6 was made by the following method.

50 wt % of PVDF relative to carbon was added to carbon carrying 40 wt %of Pt. Further, NMP in the amount of 20 times in weight of carbon wasslowly added, and they were mixed uniformly. The mixture was coated atintervals of 250 μm by blade coating, and dried by vacuum heating at160° C. for 8 hours. Then, while heated and maintained at 120° C., itwas pressed with the pressure of 300 kg/cm³ for three minutes. Thus, thecounter electrode 6 made of carbon carrying Pt and binder polymer.

Further, 2 g of lithium iodide (LiI), 5 g of 1-propyl-2,3-dimethylimidazolium iodide, 0.5 g of iodide (12) and 2 g of 4-tert-butylpyridine were dissolved in 30.5 g of acetonitrile to prepare theelectrolytic solution.

The obtained electrolyte liquid was instilled onto the metal oxidesemiconductor layer 3, and the counter electrode 6 was brought onto itvia a silicon rubber spacer (30 μm thick). Thus, the due-sensitizedsolar cell was obtained.

EXAMPLE 2

A dye-sensitized solar cell was prepared in the same manner as Example 1by using carbon carrying 40 wt % of Pt and Ru in the ratio of 1:1 inweight, and its I-V characteristics were evaluated.

EXAMPLE 3

A dye-sensitized solar cell was prepared in the same manner as Example 1excepting that the pressing for making the counter electrode 6 wascarried out at normal temperatures, and I-V characteristics of the solarbattery were evaluated.

EXAMPLE 4

A dye-sensitized solar cell was prepared in the same manner as Example 1except that the counter electrode was prepared as explained below, andI-V characteristics of the solar battery were evaluated.

200 wt % of water, 300 wt % of IPA and 30 wt % of HEC (hydroxyethylcellulose) respectively relative to carbon were added to carbon carrying40 wt % of Pt, and they were mixed together uniformly. Then, the mixturewas coated by blade coating at 250 μm intervals, and it was dried byheating at 80° C. for 8 hours. Then, while heated and maintained at 120°C., it was pressed with the pressure of 300 kg/cm³ for three minutes.Thus, the counter electrode 6 made of carbon carrying Pt and binderpolymer.

EXAMPLE 5

A dye-sensitized solar cell was prepared in the same manner as Example 4by using carbon carrying 40 wt % of Pt and Ru in the ratio of 1:1 inweight, and its I-V characteristics were evaluated.

EXAMPLE 6

A dye-sensitized solar cell was prepared in the same manner as Example 4excepting that the pressing for making the counter electrode 6 wascarried out at normal temperatures, and I-V characteristics of the solarbattery were evaluated.

EXAMPLE 7

A dye-sensitized solar cell was prepared by using a gel electrolyte, andits I-V characteristics were evaluated.

Electrolytic solution was prepared by dissolving 2 g of lithium iodide(LiI), 5 g of 1-propyl-2,3-dimethyl imidazolium iodide, 0.5 g of iodide(12) and 2 g of 4-tert-butyl pyridine were dissolved in 30.5 g of gammabutyrolactone. 150 g of dimethyl carbonate was added to the electrolyticsolution as a diluent, and the solution was heated to 70° C. After that,8 g of poly(vinylidene fluoride-hexafluoropropylene) copolymer havingthe molecular weight of 300,000 was dissolved to obtain a sol gelelectrolyte precursor. The poly(vinylidene fluoride-hexafluoropropylene)copolymer was copolymer containing vinylidene and hexafluoropropylene by95:5.

The sol gel electrolyte precursor was coated by blade coating on a metaloxide semiconductor layer 3 formed on a SnO₂-coated ITO and havingabsorbed a sensitizing dye in the same manner as Example 1. Then, it wasdried at 50° C. for five minutes to remove dimethyl carbonate. Thus, themetal oxide semiconductor layer 3 with the gel electrolyte was obtained.

The metal oxide semiconductor layer 3 with the gel electrolyte wascombined with the counter electrode 6 and silicone rubber spacer used inExample 1 to complete the dye-sensitized solar cell.

COMPARATIVE EXAMPLE 1

A dye-sensitized solar cell was prepared in the same manner by using a100 nm thick Pt film formed by sputtering as the counter electrode, andits I-V characteristics were evaluated.

COMPARATIVE EXAMPLE 2

A dye-sensitized solar cell was prepared by making the counter electrodewas prepared in the same manner as Example 1 except that the finalpressing was omitted, and its I-V characteristics were evaluated.

Evaluation of 1-V Characteristics

Photoelectric transfer characteristics of the individual dye-sensitizedsolar cells prepared as explained above.

To evaluate the photoelectric transfer efficient, alligator clips wereconnected to the transparent substrate 1 and the counter electrode 6 ineach dye-sensitized solar cell, and the dye-sensitized solar cell wasexposed to light. Then, a current generated thereby was measured by acurrent/voltage measuring device. For lighting, a xenon lamp was used asthe light source, and the light intensity was adjusted to 100 mW/cm² oneach dye-sensitized solar cell Results of the evaluation ofphotoelectric transfer characteristics are shown in Table 1.

TABLE 1 Photoelectric Transfer Coefficient Example 1 6.1% Example 2 6.0%Example 3 5.8% Example 4 5.9% Example 5 5.8% Example 6 5.9% Example 76.0% Comparative Example 1 5.6% Comparative Example 2 5.4%

As appreciated from Table 1, solar cells prepared in Examples 1-7 usingthe counter electrode 6 made of carbon carrying Pt and binder polymerexhibit relatively high photoelectric coefficients as compared withComparative Examples 1 and 2.

Heretofore, a specific embodiment of the prevent invention has beenexplained. However, the invention is not limited to this embodiment, butcontemplated various changed or modifications based on the technicalconcept of the invention.

Fog example, numerical values, structures, shapes, materials, sourcematerials, processes, and so on, are not but mere examples, and anyother appropriate numerical values, structures, shapes, materials,source materials, processes, and so on, may be employed, if necessary.

As described above, according to the invention, since the electrodecomprises a carbon carrying a metal and a binder, the charge transfervelocity on the electrode is enhanced as compared in a device where theelectrode is made of Pt, simplex carbon, conductive polymer, or thelike. Additionally, by using this electrode as the counter electrode,for example, it is possible to enhance the photoelectric transfercharacteristics of the photoelectric transfer element of adye-sensitized solar cell, for example.

1. A photoelectric transfer element comprising an electrode composed ofa carbon carrying a metal and a binder.
 2. A photoelectric transferelement according to claim 1 wherein the carbon is needle-like carbon,fullerene, carbon nanotube or electrically conductive carbon black.
 3. Aphotoelectric transfer element according to claim 1 wherein the metal isat least one kind of metal selected from the group consisting of Pt, Ru,Co, Ti, Ni, Al and Au.
 4. The electrode according to claim 1 wherein thespecific surface area of the carbon is equal to or larger than 100 m²/g.5. The electrode according to claim 1 wherein the specific surface areaof the carbon is equal to or larger than 300 m²/g.
 6. The electrodeaccording to claim 1 wherein the amount of the metal carried by thecarbon is equal to or more than 5 weight percent of the carbon.
 7. Thephotoelectric transfer element according to claim 1 wherein atransparent conductive substrate and said electrode as a counterelectrode are opposed to each other, and a semiconductor layer and anelectrolyte layer are interposed between the substrate and theelectrode.
 8. A photoelectric transfer element according to claim 1wherein the electrically conductive substrate is made of glass, apolymer film or a metal.
 9. The photoelectric transfer element accordingto claim 1 wherein the element is a dye-sensitized solar cell.